Composite shaft

ABSTRACT

A filament wound composite fibre reinforced polymer shaft comprising helical wound fibres, the shaft having at least one hole perpendicular to an axis of the shaft; wherein fibre paths of the helical wound fibres divert around the hole. The hole can be used as an attachment point to connect the shaft to other parts, e.g. by means of a pin passed directly through the hole. The amount of metal used in this type of connection can be significantly reduced compared to using metal end fittings, thus greatly reducing cost and weight of the whole system. Fibres are diverted around the hole rather than the hole being cut through the fibres which would reduce the strength of the shaft as a whole. By diverting the fibres around the hole, the fibres retain their load bearing properties and the strength of the shaft is maintained even in the presence of the hole.

FOREIGN PRIORITY

This application claims priority to European Patent Application No.17164414.9 filed Mar. 31, 2017, the entire contents of which isincorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to composite shafts and joining composite shaftsto other parts, e.g. by a clevis. The disclosure has particularapplicability to tie rods and transmission shafts for aerospaceapplications, although it can also readily be applied to many otheruses.

BACKGROUND

Composite shafts are typically formed from Polymer Matrix Composites(PMCs) which comprise some form of fibre or polymer encased within amatrix such as resin. One example is Carbon Fibre Reinforced Polymer(CFRP). Filament wound structures are typically formed by windingfilaments such as carbon fibres around a mandrel in a helical fashion soas to build up a tube shaped shaft. The angle of the helical windinginfluences the properties of the shaft. For example, windingsapproaching 45 degrees have higher torsional properties and those higherthan 45 degrees have greater properties in the hoop direction. About 45degrees is generally optimal for torque transmission. Other techniquesfor manufacturing PMCs include braiding, fibre placement techniques(including AFP), prepreg wrap techniques and pultrusion methods.Composite shafts may involve several layers, with different layershaving different properties. For example, the fibre angle may be variedbetween layers to give different properties such as for bendingresistance or impact resistance.

Interfacing with PMC rods can be challenging due to the reduced strengthby volume. Standard fixings designed to interface with metal are notusually suitable and therefore the normal solution is to interface withthe PMC rod via a metal part that is fitted onto the rod. However metalparts add weight and cost to the overall part. This detracts from theweight and cost savings which are among the greatest advantages of usingPMCs in the first place.

It is desirable that the connection between the shaft and othercomponents be structurally efficient so as to minimise weight whileensuring good force transmission and good joint robustness.

SUMMARY

According to this disclosure, there is provided a filament woundcomposite fibre reinforced polymer shaft comprising helical woundfibres, the shaft having at least one hole perpendicular to an axis ofthe shaft; wherein fibre paths of the helical wound fibres divert aroundthe hole.

Viewed from an alternative aspect, this disclosure provides a filamentwound composite fibre reinforced polymer shaft having at least one holeformed perpendicular to the axis of the shaft formed by diverting thehelical fibre paths around the hole.

The hole in the filament wound shaft can be used as an attachment pointto connect the shaft to other parts, e.g. by means of a pin passeddirectly through the hole. The amount of metal used in this type ofconnection can be significantly reduced compared to the metal endfittings used previously, thus greatly reducing cost and weight of thewhole system.

The fibres are diverted around the hole rather than the hole simplybeing cut through the fibres. Cutting the fibres would significantlyreduce the strength of the shaft as a whole as the cut would affect alarge number of fibres in multiple layers, reducing the tensile strengththat is provided by the fibres. By diverting the fibres around the hole,the fibres retain their load bearing properties and the strength of theshaft is maintained even in the presence of the hole (it will beappreciated that there may be some weakening of the structure in thevicinity of the hole, but not so much as to make the part unusable).

The filament winding process lays the fibres such that they would besubstantially parallel in the vicinity of the hole, but for thediversion around the hole. Thus, after the diversion, the fibrescontinue in the direction they would have taken if the hole were notpresent. The shaft may be terminated at a point axially removed from thehole, e.g. by cutting through the shaft, but the fibres still all runcontinuously from one end of the shaft to the other and thus transmitthe required forces.

Filament winding is better for this technique than for example braidingas individual fibre placement is possible around the hole or otherfeature to make a non-axisymmetric part, this would not be possible withbraiding as all of the fibres are manipulated together.

The main body of the composite shaft is formed by winding fibreshelically around a mandrel thus forming a hollow shaft once the mandrelis removed. A single fibre may be wound around the mandrel as it isdisplaced axially back and forth parallel to the mandrel axis such thatone fibre traverses the length of the mandrel multiple times. However,typically the resulting shaft will be cut at one or both ends afterforming, such that each lengthwise traverse ends up being a singlefibre. Regardless of whether or not such cutting is performed, eachtraverse of the shaft axis can effectively be considered as a singlefibre and that is how they are treated in the remainder of thisdisclosure. Thus each of these fibres forms a helix around the shaftaxis (and thus also around the mandrel axis). Many of these fibres willbe unaffected by the hole as their helical path does not intersect thehole. These fibres will simply follow the same path that they would havefollowed if the hole were not being formed. However, fibres which wouldhave intersected the hole will be displaced to one or other side of thehole, such that the path of these fibres deviates from the path thatthey would have taken if the hole were not being formed. Preferably thefibre path of each fibre diverted around the hole forms an arc aroundthe hole of no more than 200 degrees, preferably an arc of no more than180 degrees. It will be appreciated that each fibre must pass either toone side of the hole or to the other side of the hole. Ideally, eachfibre will be diverted to the side of the hole that requires the leastdivergence of its path and will thus contact the hole along an arc of nomore than 180 degrees. Indeed most fibres that are incident upon thehole at a lateral offset from the hole centre will contact the holealong a much smaller arc. However, it will be appreciated that someprocess variation such as vibration or manufacturing irregularities inthe forming machinery may result in a small number of fibres beingdisplaced to the less efficient side of the hole and thus a small numberof these fibres may contact the hole along an arc greater than 180degrees. However, these should be kept to a minimum and are preferablyentirely excluded.

It will be appreciated that the fibres of the shaft may have anon-axisymmetric pattern due to the divergence of the fibres around thehole.

Although a single hole may be used to form a useful join with anothercomponent in some implementations, in the majority of connections asymmetrical joint is preferred and thus the shaft preferably comprisesat least two holes each perpendicular to the axis of the shaft andwherein around each hole fibre paths of the helical wound fibres divert.In other words, the shaft preferably comprises at least two holes eachformed perpendicular to the axis of the shaft and each formed bydiverting the helical fibre paths around the respective hole. The arclength considerations described above preferably apply to both (orfurther) holes as they are preferably formed by the same process. Thisdisclosure is not limited to two holes and may incorporate any number ofholes formed in this way, e.g. three or four holes may be formed forparticular joints. Additionally it may be desirable to form twoconnections on the shaft, one at each end. Each connection may have twoor more holes, thus requiring four or more holes to be formed in theshaft.

The two holes of a joint may be non-coaxial and will be formed accordingto the type of joint that is required for a specific part and/or aspecific connection type. However, in some particularly preferredexamples the two holes are coaxial. Coaxial holes allow for a single rodor pin to be passed through both holes at once, thus making use of thestructural strength of both sides of the shaft at once. In preferredexamples the two holes form a clevis. A clevis is a yoke or fork shape(typically U-shape) with holes in each end through which a pin or boltmay be run so that the clevis can be used as a fastening device. Thusthe two holes formed in a hollow composite shaft provide the holesthrough which a pin or bolt may be inserted so that the end of thehollow shaft can be used directly as a fastening device.

The filament winding process lays fibres around a mandrel in a helicalpattern, typically with a carriage (fibre holder) passing axially backand forth along the axis of a mandrel while the mandrel rotates. Thecarriage and the mandrel are controlled in accordance with a programspecific to the part being manufactured. In order to retain the maximumtensile strength of the fibres, the fibres are preferably laid past thehole in the axial direction before the axial direction is changed byreversing the direction of the carriage. The distance that the fibresare laid past the hole will depend on several factors such as thediameter of the shaft, the diameter of the hole, the type of fibres andresin, etc. However, to ensure that the fibres pass the hole withoutchanging axial direction and have sufficient distance to regroup afterthe diversion around the hole, it is preferred that the fibres pass thehole without changing axial direction for a distance of at least onehole diameter. In this way the hole is axially spaced from an end regionin which the fibres change axial direction as part of the normalfilament winding process.

In some examples, some fibres may be turned around the hole, i.e. theymay change axial direction by turning around the hole. Other fibres thatdo not intersect the hole may turn around in the conventional manner,e.g. at an axial position distanced from the hole.

According to a further aspect of this disclosure, there is provided amethod of forming a composite fibre reinforced polymer shaft having atleast one hole formed perpendicular to the axis of the shaft,comprising: providing a mandrel having a projection at the locationwhere the hole is to be formed; and using a filament winding process tolay fibres in a helical fashion around the mandrel such that they aredisplaced around the projection in the location where the hole is to beformed.

This method of forming a shaft with a hole uses a physical structure asan obstruction to push the fibres out of the way during the windingprocess. As the projection takes up the space that will eventually formthe hole in the finished product, the fibres are natural diverted out ofthe way by being forced to divert around the projection. However thenatural position of the fibre is retained as far as possible by thecontrol of the winding process, i.e. control of the mandrel and thecarriage. These can operate substantially as they would have done in theabsence of any holes, thus essentially ignoring the hole formingprocess. After fibres have been diverted by the projection(s), thefibres will naturally be drawn back onto their original paths.

In some examples, additional strengthening fibres may be added to theshaft in the vicinity of the hole. For example, where the fibres havebeen diverted around the hole, there may be small gaps or weaknesses inthe shaft structure. Additional windings may be used to overlay thesegaps or to add strength to the other windings. Some examples of suchadditional windings are as follows:

1) hoop wound reinforcement fibre wound adjacent to the hole on eitherone or both axial sides of the hole.

2) circumferential hole reinforcement fibre wound around thecircumference of the hole.

3) in the case of two or more holes at one axial end of the shaft,reinforcement lashing wrapped around two holes (preferably adjacentholes). Such lashing may be cross-lashed such that it is wrapped in afigure of eight pattern around the two holes.

It will be appreciated that as part of the process of forming a finishedproduct, a matrix material such as resin must be provided around thefibres. This may be achieved in any of the usual manners such as bytowing the fibres through a resin bath during the fibre placement or bysoaking the wound fibres and mandrel in a resin bath after the windingprocess. In some examples the fibres are prepreg fibres, i.e. fibresprovided with resin already coated thereon. The prepreg fibres aretypically slightly stickier than fibres drawn through a resin bath andthus will tend to stick to each other more during the fibre winding.Thus when a fibre contacts the projection and is diverted away from itsnormal path, the parts of the fibre that have already been placed incontact with the underlying fibres or mandrel will be displaced less,thus encouraging the diverging path to be closer to an arc around thehole/projection. In other preferred examples the fibres are wet wound,e.g. towed through a resin bath during placement. Wet fibres are moreeasily diverted around an obstruction due to not being as sticky.

The projection preferably has a taper that narrows away from themandrel. In some preferred examples it tapers to a point. The projectionmay have a cylindrical base adjacent to the mandrel with a tapered partprovided above it. The tapering helps to ensure that fibres are dividedinto two groups falling either side of the projection without the riskof any fibres getting caught on top of the projection. The projectionmay be parallel-sided immediately adjacent to the mandrel or it may havea taper towards the mandrel to facilitate or enable removal of theprojection from the mandrel after curing.

As discussed above, two or more holes may be desirable in certainimplementations, e.g. two opposite holes to form a clevis. Thuspreferably at least two projections are provided on the mandrel,preferably diametrically opposite one another.

It will be appreciated that after forming the product, the mandrel ispreferably removed from the shaft. Thus the method preferably furthercomprises: removing the projection(s); and removing the mandrel. It willbe appreciated that this takes place after application of the resin andcuring of the resin. The mandrel and projections may all be removed in asingle process, e.g. in the case of a dissolvable mandrel or otherdeformable mandrel. However, if the mandrel is rigid and non-dissolvablethen the projections are preferably removed first, followed by removingthe rest of the mandrel along the main axis of the shaft.

Where two diametrically opposite projections are used, these may beprovided as separate projection each attached to the mandrel or they maybe formed as a single pin that passes through a hole formed in themandrel, thus ensuring accurate alignment of the holes in the endproduct.

As discussed above, preferably the filament winding process changes theaxial direction of the fibres at an axial position spaced apart from theprojection(s). This ensures that the fibres run past the holes and canprovide tension around the holes and along the full length of the shaft.

As fibres are diverted around the hole, it will be appreciated thatthere will be a build up in the radial direction of fibres around thecircumference of the hole as the diverted fibres are laid on top ofother diverted fibres and on fibres that are undiverted. This build upmay be removed after the product has been formed (i.e. after curing) soas to provide a consistent diameter along the shaft. Thus preferably themethod further comprises a step of machining the shaft in the region ofthe hole to remove excess material. While this may result in thesevering of a few fibres, it will not cut through so many that thestructural strength of the product is affected. If this is of concernthen it will be appreciated that this machining is not essential.

BRIEF DESCRIPTION OF DRAWINGS

One or more non-limiting examples will now be described, by way ofexample only, and with reference to the accompanying figures in which:

FIG. 1 shows a first example of fibre placement around a hole;

FIG. 2 shows a second example of fibre placement around a hole;

FIG. 3 illustrates the contact angle of fibres adjacent to a hole;

FIG. 4 illustrates two fibre reinforcement techniques;

FIG. 5 illustrates an additional fibre reinforcement technique and useof a pin for fibre turnaround; and

FIG. 6 shows a clevis formed in a hollow shaft comprising two holes.

FIG. 1 shows a composite filament wound shaft 1. A hole 2 is formed inthe shaft 1 perpendicular to the shaft axis. The paths of certain fibres3 are shown in FIG. 1. It will be appreciated that only a reduced numberof fibres 3 are shown for illustrative purposes and so that the pathscan be distinguished. In reality many more fibres are used, spaced muchcloser together.

It can be seen in FIG. 1 that the fibres 3 have been diverted around thehole such that they follow a different path from the path that theywould have taken if they had been laid down unhindered. In a filamentwinding process the fibre is supplied through the carriage which ispassed axially back and forth along the length a mandrel while themandrel is rotated. The fibre thus forms a helical path around themandrel with the angle of the helix being determined by the traversingspeed of the carriage relative to the rotation speed of the mandrel.Several layers may be deposited on top of each other and these layersmay have different fibre angles. Adjacent fibres in any layer generallyhave the same angle such that they run substantially parallel pathsaround the shaft/mandrel. In this context, parallel paths means pathswith the same helix angle, but with an axial displacement along theshaft axis. However, as can be seen in FIG. 1, in this example, thefibres 3 in the vicinity of the hole 2 have been displaced such thatthey no longer run in parallel paths in the vicinity of the hole 2, butare diverted from their normal paths so as to form the hole. Fibres 3 aand 3 b would, if no hole were being formed, have been deposited insubstantially parallel helical paths. However, to form the hole 2,fibres 3 a have been deflected to one side of the hole 2 and fibres 3 bhave been deflected to the opposite side of the hole 2. Similarly,fibres 3 c and 3 d would, if no hole were being formed, have beendeposited in substantially parallel helical paths. However, to form thehole 2, fibres 3 c have been deflected to one side of the hole 2 andfibres 3 d have been deflected to the opposite side of the hole 2.Fibres 3 a and 3 b are typically deposited in one direction (e.g. leftto right in the figure) while fibres 3 c and 3 d are deposited in theother direction (e.g. right to left in the figure) with the mandrelrotating in the same direction throughout. Thus the fibres 3 a, 3 b, 3 cand 3 d are diverted around four different tangential points of thehole. In FIG. 1, the hole 2 is illustrated by an ideal circle, althoughit will be appreciated that this ideal shape may not be achieved inpractice simply with the windings shown in FIG. 1.

The fibre paths 3 a-3 d shown in FIG. 1 show paths that may be formedwhen the fibres have a reasonable degree of freedom to move as they aredeposited such that is the fibres 3 a-3 d are deflected, the path isdeflected across a relatively long distance. This may be the case withwet wound fibre winding techniques such as towing through a resin bathas the fibres are less sticky and thus do not immediately adhere tounderlying layers during deposition.

FIG. 2 shows some alternative deflected fibre paths 3 e and 3 f whichmay be more typical of winding with prepreg fibres as these are moresticky and will adhere to the underlying layer resulting in pathdeflection being closer to the hole 2 and contacting the hole 2 along alonger path. Fibres 3 e pass to one side of the hole 2 while fibre 3 fpasses to the other side of hole 2 as they approach the hole fromdifferent sides of its centreline, each passing on the side of hole 2that will result in the shortest path around the hole 2. Again, hole 2is shown in an ideal circular form which will not be achieved inpractice. FIG. 2 also shows some undeflected fibres 3 g which do notpass adjacent to the hole 2 and thus are not deflected from their normalhelical path.

FIG. 3 illustrates the difference in path shape in more detail. Asexamples, the paths of fibres 3 b and 3 e passing around hole 2 areshown. Fibre 3 e contacts the ideal circular hole shape 2 along a largerangle a than fibre 3 b which contacts the ideal circular hole shape 2along an angle b. Both angles a and b are significantly smaller than 180degrees, i.e. the fibres are not wound all the way around the hole 2,but are merely deflected around it.

FIG. 4 illustrates the mandrel 4 that is used for forming the shaft 1with hole 2. The mandrel 4 has pins (or projections) 5 projecting fromit substantially perpendicular to its axis. In this example two pins (orprojections) 5 are shown, but a single pin (or projection) 5 could beused, or a greater number of pins or projections 5 could be used. Thebasic filament winding process can be used in the same manner as fornormal shafts without holes. However the pins (or projections) 5naturally deflect the fibres 3 as they are laid down on the mandrel 4.As shown in FIG. 4, each pin (or projection) 5 has a tapered section 6that tapers away from the mandrel 4 and in this example tapers to apoint that ensures fibres are deflected either to one side or the otherof pin (or projection) 5 as they are laid down. The pins (orprojections) 5 also taper very slightly towards the mandrel in thesection 7 adjacent to the mandrel 4 as this helps with removal of thepins (or projections) 5 later in the process. The programme thatcontrols the filament winding process will preferably position thefibres around the pin rather than just relying on them being deflectedaround it.

For clarity, FIG. 4 does not shown the normal helical fibres 3 that areillustrated in FIGS. 1 and 2. However, FIG. 4 does illustrate twotechniques for adding strengthening fibres around the hole 2. The firsttechnique is simply to wind fibre 8 around the pin 5, thus depositingcircular loops of fibre around the circumference of the hole 2, fillingin or strengthening gaps that may have been formed in the main filamentwinding process and building up the profile of the shaft 1 in the regionof the hole 2. The second technique illustrated in FIG. 4 is across-lash 9 which is wound around two pins 5, each of which forms ahole 2 in the shaft 1. In the example shown in FIG. 4, the two pins (orprojections) 5 are coaxial and diametrically opposite one another on theshaft 1 such that together they will form a clevis at the end of theshaft 1. The cross-lash fibre 9 passes around one pin 5 in one sense(clockwise or anticlockwise) when viewed towards the mandrel, thenpasses around the other pin 5 in the opposite sense, also viewed towardsthe mandrel, before returning to pass around the first pin 5 in the samesense as before, thus forming a figure of eight pattern that crossesover itself between the pins 5 and which can be repeated a number oftimes to lay down a suitable volume of fibre. It will be appreciatedthat a similar cross lash 9 may also be formed around the same two pins5 on the opposite side of the mandrel for symmetry and balancing. Itwill also be appreciated that a non-crossed lash may also be performedby passing the fibre in the same sense around both pins and not crossingthe fibre over itself between the pins 5.

FIG. 5 shows a further fibre reinforcement technique for the hole 2 bywinding hoop fibre 10 around the mandrel 4 on one or both axial sides ofthe pins (or projections) 5 and thus on one or both sides of the hole 2(both sides are illustrated in FIG. 5).

FIG. 5 also shows how the pins 5 may be used to turn helical woundfibres around the pin 5 when the carriage changes direction. If thenatural helical path of the fibre would bring it into contact with thepin 5, it can be turned neatly around the pin 5, thus changing directionof movement along the mandrel axis.

It will be appreciated that the techniques illustrated in FIG. 4 willrequire oscillating the mandrel 4 back and forth while moving thecarriage back and forth. The techniques illustrated in FIG. 5 can beperformed while the mandrel 4 continues to rotate in the same direction.

The techniques shown in FIGS. 4 and 5 may of course be used either aloneor in combination so as to provide the optimal reinforcement for aparticular application.

To form a hollow shaft with a hole according to the techniques describedhere, a mandrel 4 is provided with at least one pin 5 projectingradially outwardly at the position at which the hole 2 is desired. Thefilament winding process is then performed as for a normal shaft windingprocess, but with the fibres being deflected by the pin(s) 5 so that nofibre is laid in the region where the hole 2 is desired. Beneficially,the hole is formed without cutting through the fibres in the region ofthe hole 2, thus all fibres provide strength across the hole, improvingthe overall properties of the shaft 1. Additional fibre strengtheningtechniques may then be applied around the pin(s) 5 (i.e. in the regionof the hole(s) 2) so as to add additional strength to the shaft in thevicinity of the holes 2. Resin is applied using known techniques, suchas using resin baths and/or prepreg fibres and the shaft is cured, againusing known techniques. The pins 5 and mandrel 4 are then removed,leaving a shaft 1 with one or more holes 2 formed therein. Where thepins 5 are fixedly mounted to the mandrel 4, they must normally beremoved first before the mandrel 4 can be removed. However, it will beappreciated that other techniques such as dissolvable mandrels may alsobe used in which case the order of removal is not important.

FIG. 6 shows a clevis 11 formed in a hollow shaft 1 comprising two holes2, each formed according to the techniques described above. A rod 12 isshown that can be passed through the two holes 2 of the clevis 11 (andalso through any additional connecting structure inserted inside theshaft 1 between the holes 2 or provide outside of the shaft 1 around theholes 2), e.g. for connection of other devices or equipment.

1. A filament wound composite fibre reinforced polymer shaft comprisinghelical wound fibres, the shaft having at least one hole perpendicularto an axis of the shaft; wherein fibre paths of the helical wound fibresdivert around the hole.
 2. A shaft as claimed in claim 1, wherein thefibre path of each fibre diverted around the hole forms an arc aroundthe hole of no more than 200 degrees, preferably an arc of no more than180 degrees.
 3. A shaft as claimed in claim 1, comprising at least twoholes each perpendicular to the axis of the shaft and wherein aroundeach hole fibre paths of the helical wound fibres divert.
 4. A shaft asclaimed in claim 3, wherein the two holes are coaxial.
 5. A shaft asclaimed in claim 3, wherein the two holes form a clevis.
 6. A shaft asclaimed in claim 1, wherein the fibres pass the hole without changingaxial direction for a distance of at least one hole diameter.
 7. A shaftas claimed in claim 1, further comprising additional strengthening fibreadded to the shaft in the vicinity of the hole.
 8. A shaft as claimed inclaim 7, wherein said additional strengthening fibre comprises hoopwound reinforcement fibre wound adjacent to the hole on one or bothaxial sides of the hole.
 9. A shaft as claimed in claim 7, wherein saidadditional strengthening fibre comprises circumferential holereinforcement fibre wound around the circumference of the hole.
 10. Ashaft as claimed in claim 7, wherein the shaft comprises two or moreholes and wherein said additional strengthening fibre comprisesreinforcement lashing wrapped around two of said holes.
 11. A shaft asclaimed in claim 10, wherein said lashing may be cross-lashed such thatit is wrapped in a figure of eight pattern around said two holes.
 12. Amethod of forming a composite fibre reinforced polymer shaft having atleast one hole formed perpendicular to the axis of the shaft,comprising: providing a mandrel having a projection at the locationwhere the hole is to be formed; and using a filament winding process tolay fibres in a helical fashion around the mandrel such that they aredisplaced around the projection in the location where the hole is to beformed.
 13. A method as claimed in claim 12, wherein the projectioncomprises a taper that narrows towards the mandrel.
 14. A method asclaimed in claim 12, further comprising: removing the projection(s); andremoving the mandrel.
 15. A method as claimed in claim 14, wherein thefilament winding process changes the axial direction of the fibres at anaxial position spaced apart from the projection(s).